Workflow: don't change REQUEST when passing transition

The new workflow implementation from df85ef46 (ERP5Workflow: DC Workflows
are now ERP5 objects (!1378)., 2020-11-18) had a different behavior in
the implementation of _executeTransition, it was setting all kwargs
from the transition in REQUEST. The reasons for this are unclear, it
seem to come from the first Workflow implementation, 81784db4 (Initial
Import of ERP5Workflow porduct., 2010-10-15)

The side effects are that calling a workflow method, for example:

  document.edit(key=value)

would also alter the global request to set key=value in REQUEST, for the
rest of the current request.

It was causing subtle issues, especially in tests where the REQUEST
lifetime is long.